Conventional spine cages or implants are typically characterized by a kidney bean-shaped body comprising a hydroxyapatite-coated surface provided on the exterior surface for contact with adjacent vertebral segments or endplates which are shown in FIG. 1. A conventional spine cage with flat endplates is typically inserted posterolaterally proximate to the neuroforamen of the distracted spine after a trial implant creates a pathway. Optionally two parallel externally threaded conduits are inserted anteriorly to achieve lumbar arthrodesis. The implants are often of constant diameter whereas the L5-S1 disc space is trapezoidal, thus a ‘flat back’ syndrome may be iatrogenically created. Generally spine intradiscal implants are for lumbar fusion or cervical motion preservation, while a separate system of rods and screws corrects alignment.
With the novel UECs disclosed herein, additional options include fusion throughout the spinal column, and deformity angular correction.
Existing devices for interbody stabilization have important and significant limitations. Among the limitations are an inability to expand and distract the endplates. Consequently, if a cage that is “to small” is inserted it can ‘rattle around and never heal’. If the static cage is too big, it can injure adjacent nerves or destabilize the spine via end plate resection or subsidence.
Current devices for interbody stabilization include static spacers composed of titanium, PEEK, and high performance thermoplastic polymer produced by VICTREX, (Victrex USA Inc, 3A Caledon Court, Greenville, S.C. 29615), carbon fiber, or resorbable polymers. Current interbody spacers may not maintain interbody lordosis and can contribute to the formation of a straight or even kyphotic segments and the clinical problem of “flatback syndrome.” Separation of the endplates increases space available for the neural elements, specifically the neural foramen. Existing static cages do not reliably improve space for the neural elements. Therefore, what is needed is an expanding cage that will increase space for the neural elements posteriorly between the vertebral bodies, or at least maintain the natural bone contours to avoid neuropraxia (nerve stretch) or encroachment.
U.S. Pat. No. 7,985,256, filed Sep. 26, 2006 and titled “Selectively Expanding Spine Cage, Hydraulically Controllable in Three Dimensions for Enhanced Spinal Fusion”, and U.S. Pat. No. 7,819,921, filed Oct. 31, 2007 and titled “Linearly expanding spine cage for enhanced spinal fusion”, both provide detailed background on expanding spine cages.
The cages disclosed in U.S. Pat. No. 7,985,256 above are restricted to use with hydraulics, and lumbar fusion. The cage disclosed in U.S. Pat. No. 7,819,921 allows for trapezoidal linear expanding, not uniform expansion, thus a trapezoidal L5 cage as disclosed therein will preserve natural lumbar lordosis. The disclosed cage was never developed. It is intended for use as two (2) parallel linearly expanding split conduits inserted anteriorly for lumbar fusion.
In contrast, the UEC cages disclosed herein expands either uniformly, or at either end proximally or distally. Given the adjustment option the surgeon can correct angulation deformity with the novel UEC.
Another problem with conventional devices of interbody stabilization includes poor interface between bone and biomaterial. Conventional static interbody spacers form a weak interface between bone and biomaterial. Although the surface of such implants is typically provided with a series of ridges or coated with hydroxyapetite, the ridges may be in parallel with applied horizontal vectors or side-to-side motion. That is, the ridges or coatings offer little resistance to movement applied to either side of the endplates. Thus, nonunion is common in allograft, titanium and polymer spacers, due to motion between the implant and host bone. Conventional devices typically do not expand between adjacent vertebrae. Since the UEC expands under surgeon control, the visible, palpable ‘goodness of fit’ setting can ideal lock opposing vertebral endplates at the time of surgery. As healing accrues, the implants become inert. Since no motion equates with no pain, clinical results are improved with UECs.
Therefore, what is needed is a way to expand an implant to develop immediate fixation forces that can exceed the ultimate strength at healing, with improved abilities to enable disc space fixation solidarity while correcting spine angular deformity. Such an expandable implant ideally will maximize stability of the interface and enhance stable fixation. The immediate fixation of such an expandable interbody implant advantageously will provide stability that is similar to that achieved at the time of healing. Such an implant will have valuable implications enhancing early post-operative rehabilitation for the patient.
Another problem of conventional interbody spacers is their large diameter requiring wide exposure. Existing devices used for interbody spacers include structural allograft, threaded cages, cylindrical cages, and boomerang-shaped cages. Conventional devices have significant limitation with regard to safety and efficacy. Regarding safety of the interbody spacers, injury to neural and aortic elements may occur with placement from an anterior or posterior approach. A conventional spine cage lacks the ability to expand, diminishing its fixation capabilities. Prior attempts to preserve lumbar motion have failed by extrusion of the implant after implantation. The risks to neural elements are primarily due to the disparity between the large size of the cage required to adequately support the interbody space, and the small space available for insertion of the device, especially when placed from a posterior or transforaminal approach. Existing boomerang cages are shaped like a partially flattened kidney bean. Their implantation requires a wide exposure and potential compromise of vascular and neural structures, both because of their inability to enter small and become larger, and due to the fact that their insertion requires mechanical manipulation during insertion and expanding of the implant. Once current boomerang implants are prepared for insertion via a trial spacer to make a pathway toward the anterior spinal column, the existing static cage is shoved toward the end point with the hope that it will reach a desired anatomic destination. Given the proximity of nerve roots and vascular structures to the insertion site, and the solid, relatively large size of conventional devices, such constraints predispose a patient to foraminal (nerve passage site) encroachment, and possible neural and vascular injury.
Therefore, what is needed is a minimally invasive expanding spine cage that is capable of insertion with minimal invasion into a smaller aperture. Such a minimally invasive spine cage advantageously could be expanded with completely positional control or adjustment in three dimensions. What is also needed is a smaller expanding spine cage that is easier to operatively insert into a patient with minimal surgical trauma in contrast to conventional, relatively large devices that create the needless trauma to nerve roots in the confined space of the vertebral region. Existing interbody implants have limited space available for bone graft. Adequate bone graft or bone graft substitute is critical for a solid interbody arthrodesis. It would be desirable to provide an expandable interbody cage that will permit a large volume of bone graft material to be placed within the cage and around it, to fill the intervertebral space. Additionally, conventional interbody implants lack the ability to stabilize endplates completely and prevent them from moving. Therefore, what is also needed is an expanding spine cage wherein the vertebral end plates are subject to forces that both distract them apart, and hold them from moving. Such an interbody cage would be capable of stabilization of the motion segment, thereby reducing micromotion, and discouraging pseudoarthrosis (incomplete fusion) and pain.
Ideally, what is needed is a spine cage or implant that is capable of increasing its expansion in height and angle, spreading to a calculated degree. Furthermore, what is needed is a spine cage that can adjust the amount of not only overall anterior posterior expansion, but also medial and lateral variable expansion so that both the normal lordotic curve is maintained, and adjustments can be made for scoliosis or bone defects. Such a spine cage or implant would permit restoration of normal spinal alignment after surgery and hold the spine segments together rigidly, mechanically, until healing occurs.
What is also needed is an expanding cage or implant that is capable of holding the vertebral or joint sections with increased pullout strength to minimize the chance of implant fixation loss during the period when the implant is becoming incorporated into the arthrodesis bone block.